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Fatigue risk index (FRI)

A fatigue risk index (FRI) is a method for estimating fatigue-related exposure associated with a shift pattern or roster, using structured inputs such as shift times, duration, rest between duties, breaks, and workload.

In the UK, widely referenced approaches build on research published by HSE as Research Report RR446 — The development of a fatigue / risk index for shiftworkers (2006). HSE’s fatigue guidance notes that risk assessment of working hours may include tools such as a fatigue risk index. Use of any index remains an organisational choice, configured to context and reviewed by competent persons.

FRI methods appear in transport and other shift-work sectors to support roster review and dialogue between planners, safety teams, and workforce representatives. They are decision-support tools, not substitutes for fatigue risk assessment or management judgement.

Section titled “What an FRI produces: two related outputs”

HSE’s fatigue/risk index framework produces two complementary scores. Both depend on the same underlying schedule inputs but emphasise different aspects of risk.

Fatigue Index (FI)

Section titled “Fatigue Index (FI)”

The Fatigue Index is typically expressed on a scale up to 100. In the HSE framework, it represents a modelled estimate of the probability that a worker would experience very high sleepiness during the duty period, derived from roster timing, cumulative work history, and job/break factors.

Plain-English interpretation:

  • Higher FI → model predicts greater likelihood of severe sleepiness during the shift
  • FI describes acute fatigue likelihood for the pattern — not cumulative injury risk on its own

FI is not a direct measurement of how alert a specific person feels at work.

Risk Index (RI)

Section titled “Risk Index (RI)”

The Risk Index is typically expressed as a relative multiplier, with a reference schedule often normalised around 1.0. In the HSE framework, RI compares modelled accident/incident risk for the duty or pattern against that reference.

Plain-English interpretation:

  • RI = 1.0 → broadly comparable to the reference pattern
  • RI = 1.5 → model suggests roughly 50% higher relative risk than the reference
  • RI is comparative — meaningful when comparing options, not as an absolute accident probability

Risk and fatigue peaks can occur at different times of day in the model. HSE’s calculator guidance recommends reviewing both indices when assessing a pattern.

Three components behind the scores

Section titled “Three components behind the scores”

HSE RR446 describes three components that feed fatigue and risk calculations:

Component What it captures
Cumulative How earlier shifts and rest in the schedule build fatigue over days
Duty timing Start time, shift length, and time-of-day effects (circadian influence)
Job type / breaks Workload, vigilance demand, and break pattern within the shift

A single shift cannot usually be assessed in isolation — the cumulative component requires schedule history (previous shifts, rest days, and gaps).

What FRI can help with

Section titled “What FRI can help with”

Used appropriately, an FRI can:

  • Provide a consistent basis for comparing roster options before implementation
  • Highlight shifts or sequences with elevatively modelled fatigue exposure
  • Support evidence-led discussions during shift work and night shift planning
  • Create a documented basis for review alongside wider FRMS controls

What FRI cannot do

Section titled “What FRI cannot do”

Before using FRI outputs in decisions, read limitations of fatigue models. Critical points:

  • FRI does not measure a worker’s real-time alertness or fitness for duty
  • FRI does not predict whether a specific accident will occur
  • A low score does not guarantee safety; a high score does not automatically mean a shift must be cancelled — organisational policy and competent review determine response
  • Different methodologies and policy thresholds produce different results
  • Colour bands or trigger levels (e.g. green/amber/red) are typically organisational policy thresholds, not universal legal limits. HSE states it does not currently specify threshold scores for fatigue — organisations should aim to reduce scores so far as is reasonably practicable and seek staff feedback on how tiring patterns feel in practice.

For a plain-English comparison of the two outputs, see Fatigue Index vs Risk Index.

Any output should be interpreted by competent persons with knowledge of the operation.

Typical inputs

Section titled “Typical inputs”

Depending on methodology and software, inputs commonly include:

Input category Examples
Schedule Shift start and end date/time, consecutive duties, rest days
Shift characteristics Duration, night vs day, early start
Rest and gaps Time off between shifts
Breaks Frequency, length, continuous work periods
Job factors Workload intensity, attention demand (where the model supports them)
Travel / commute Some organisational policies include travel burden in assessment — see duty, travel, and commute

Poor input data — wrong times, missing previous shifts, unrealistic break assumptions — produces unreliable outputs regardless of model quality.

Using FRI within governance

Section titled “Using FRI within governance”

Sensible practice treats FRI as one input to a governed process:

  1. Plan — assess rosters at design stage before assignment
  2. Compare — evaluate alternatives against a baseline pattern
  3. Document — record scores, assumptions, and who reviewed them
  4. Decide — apply organisational policy, mitigations, or redesign as appropriate
  5. Reassess — when actual hours, assignments, or travel differ from plan

Infrastructure and transport organisations may apply contractual fatigue standards with defined response levels. Those standards apply to specific contractual contexts — they should not be presented as universal UK law on this site.

What this page does not cover

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This page explains FRI concepts at an educational level. It does not reproduce proprietary standard text, endorse a particular software product, or provide organisation-specific trigger thresholds. Detailed rail contractual requirements need review against primary sources by competent persons.

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References

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